Discovery of superoxide reductase: an historical perspective.

International audienceFor more than 30 years, the only enzymatic system known to catalyze the elimination of superoxide was superoxide dismutase, SOD. SOD has been found in almost all organisms living in the presence of oxygen, including some anaerobic bacteria, supporting the notion that superoxide is a key and general component of oxidative stress. Recently, a new concept in the field of the mechanisms of cellular defense against superoxide has emerged. It was discovered that elimination of superoxide in some anaerobic and microaerophilic bacteria could occur by reduction, a reaction catalyzed by a small metalloenzyme thus named superoxide reductase, SOR. Having played a major role in this discovery, we describe here how the concept of superoxide reduction emerged and how it was experimentally substantiated independently in our laboratory

Superoxide reductase from Desulfoarculus baarsii.

International audienceSuperoxide radical (O2.-) is the univalent reduction product of molecular oxygen and belongs to the group of the so-called toxic oxygen derivatives. For years the only enzymatic system known to catalyze the elimination of superoxide was the superoxide dismutase (SOD), which catalyzes dismutation of superoxide radical anions to hydrogen peroxide and molecular oxyge

Photochemical processes observed during the reaction of superoxide reductase from Desulfoarculus baarsii with superoxide: re-evaluation of the reaction mechanism.

International audienceSuperoxide reductase SOR is an enzyme involved in superoxide detoxification in some microorganisms. Its active site consists of a non-heme ferrous center in an unusual [Fe(NHis)(4) (SCys)(1)] square pyramidal pentacoordination that efficiently reduces superoxide into hydrogen peroxide. In previous works, the reaction mechanism of the SOR from Desulfoarculus baarsii enzyme, studied by pulse radiolysis, was shown to involve the formation of two reaction intermediates T1 and T2. However, the absorption spectrum of T2 was reported with an unusual sharp band at 625 nm, very different from that reported for other SORs. In this work, we show that the sharp band at 625 nm observed by pulse radiolysis reflects the presence of photochemical processes that occurs at the level of the transient species formed during the reaction of SOR with superoxide. These processes do not change the stoichiometry of the global reaction. These data highlight remarkable photochemical properties for these reaction intermediates, not previously suspected for iron-peroxide species formed in the SOR active site. We have reinvestigated the reaction mechanism of the SOR from D. baarsii by pulse radiolysis in the absence of these photochemical processes. The T1 and T2 intermediates now appear to have absorption spectra similar to those reported for the Archaeoglobus fulgidus SOR enzymes. Although for some enzymes of the family only one transient was reported, on the whole, the reaction mechanisms of the different SORs studied so far seem very similar, which is in agreement with the strong sequence and structure homologies of their active sites

Fe3+-hydroxide ligation in the superoxide reductase from Desulfoarculus baarsii is associated with pH dependent spectral changes.

International audienceSuperoxide reductase (SOR) catalyzes the reduction of O2*- to H2O2. Its active site consists of a non-heme Fe2+ center in an unusual square-pyramidal [His4 Cys] coordination. Like many SORs, the electronic absorption band corresponding to the oxidized active site of the SOR from Desulfoarculus baarsii exhibits a pH-dependent alkaline transition changing from ca. 644 to 560 nm as the pH increases and with an apparent pKa of 9.0. Variants in which the conserved amino acids glutamate 47 and lysine 48 were replaced by the neutral residues alanine (E47A) and isoleucine (K48I), respectively, exhibited the same alkaline transition but at lower apparent pKa values of 6.7 and 7.6, respectively. Previous work [Nivière, V.; Asso, M.; Weill, C. O.; Lombard, M.; Guigliarelli, B.; Favaudon, V.; Houée-Levin, C. Biochemistry 2004, 43, 808-818] has shown that this alkaline transition is associated with the protonation/deprotonation of an unidentified base, B-, which is neither E47 nor K48. In this work, we show by resonance Raman spectroscopy that at basic pH a high-spin Fe3+-OH species is formed at the active site. The presence of the HO- ligand was directly associated with an absorption band maximum at 560 nm, whereas upon protonation, the band shifts to 644 nm. With respect to our previous work, B- can be identified with this high-spin Fe3+-OH species, which upon protonation results in a water molecule at the active site. Implications for the SOR catalytic cycle are proposed